In the manufacture of integrated circuits, various thin films are deposited and patterned on a semiconducting substrate. One well known deposition process is chemical vapor deposition (CVD). In general, chemical vapor deposition (CVD) is a process in which a substrate (e.g. wafer) is heated and coated with vapors of volatile chemical compounds at a temperature below the melting point of the substrate. During CVD, a precursor compound, in a vapor or gaseous state, is reduced or dissociated, in a chemical reaction on the substrate surface, thereby resulting in an adherent coating deposited on the substrate.
In semiconductor manufacture, CVD is used to produce epitaxially grown single crystal silicon by the reduction of a silicon precursor such as silicon tetrachloride (SiCl.sub.4) by a reactant gas such as hydrogen. This process is used to make epitaxial compounds such as polysilicon, silicon nitride, silicon dioxide, and both doped polysilicon and silicon dioxide. Chemical vapor deposition is also utilized in semiconductor manufacture in the deposition of various conductive films such as aluminum, copper, titanium, nichrome and platinum.
Recent advancements in semiconductor manufacture have led to increases in the density and miniaturization of microelectronic circuits. This has necessitated the use of new materials and accelerated the development of improved deposition techniques including chemical vapor deposition (CVD) for these materials. One improved CVD process for depositing high performance conductive films is known as metal organic chemical vapor deposition (MOCVD). With MOCVD, an organometallic compound is used as a precursor for the deposition material. Such organometallic precursors permit the deposition process to be performed at lower temperatures and with the incorporation of fewer impurities.
As an example, titanium nitride (TiN) films which are widely used in many advanced metallization schemes, can be deposited by MOCVD. For depositing titanium nitride, an organometallic precursor of titanium may be a dialkylamino-derivative, such as tetrakis-dimethyl-amino titanium [Ti(N(CH.sub.3).sub.2).sub.4 ] (TDMAT).
One limitation in the use of an organometallic precursors for CVD is that the majority of organometallic compounds with attributes desirable for CVD are solids. With a solid organometallic precursor, the solid phase material must be transformed directly to the vapor phase without passing through the liquid phase. This conversion from solid to vapor phase is known as sublimation . Volatility is the tendency of a solid material to pass into the vapor state at a given temperature.
For CVD, an organometallic compound must be volatile, have sufficient stability to transport to the deposition site, and decompose cleanly giving the desired material. Many organometallic compounds have the chemical stability molecular structure and reactivity which makes them ideally suited for CVD applications. The difficulty of subliming or transforming the solid material to a vapor phase, however, is a major hindrance to their implementation in a production environment.
As an example, it may be difficult to achieve a rate of sublimation or organometallic precursors that is fast enough for a production throughput. Further, it may be difficult to maintain a consistent delivery rate of a precursor gas over the course of a deposition process (i.e. the delivery rate decreases as the deposition process proceeds).
FIG. 1 illustrates a prior art method for subliming a solid precursor during a CVD deposition process. A CVD reactor 10 includes solid precursors 12 contained at the bottom of a sealed reaction chamber 14. The reaction chamber 14 is connected to a vacuum source 22. Different precursor compounds may be mixed in a ratio to provide a desired stoichiometric composition for the precursor gas. The solid precursors 12 are heated by a heat bath 16 to a temperature sufficient to cause the sublimation of the precursors 12 to a vapor state and form a precursor gas. A substrate 18 is mounted above the precursor 12. The substrate 18 is heated by a power supply 20 to a temperature which causes the precursor gas within the reaction chamber 14 to decompose and deposit onto the substrate 16.
With such a prior art CVD deposition process, it may be difficult to achieve an acceptable rate of sublimation of the precursor. In addition, it may be difficult to maintain a consistent and uniform delivery of the precursor gas throughout the deposition process. Furthermore, temperature fluctuations caused by the sublimation reaction may adversely affect the rate and uniformity of the sublimation process.
As is apparent then, there is a need in the art for improved methods and apparatus for subliming materials. Particularly in semiconductor manufacture, there is a need for improved methods and apparatus for subliming organometallic precursors during CVD at a rate fast enough to allow an acceptable production throughput. It is also desirable in semiconductor manufacture to control the sublimation process during CVD to provide a uniform supply of the precursor gas throughout the duration of the deposition process.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for subliming precursors and in particular solid organometallic precursors. It is a further object of the present invention to provide an improved method and apparatus for subliming solid organometallic precursors suitable for use in a chemical vapor deposition (CVD) process. It is yet another object of the present invention to provide an improved method and apparatus for achieving a high rate of sublimation of a solid precursor and a uniform delivery of a precursor gas. It is yet another object of the invention to provide an improved method and apparatus for subliming solid organometallic precursors that are efficient, low cost and adaptable to large scale semiconductor manufacture.